I hade a several pens of cows, pigs, sheep and chickens in a fence pen and sometimes I find pigs, sheep, etc. outside of the fenced area. I have not seen the process, but I hypothesize that it happens when they grow into adults and get glitched out of the fence.

To reproduce:
Put baby and adult animal mobs in a fence pen at least 5x5 blocks.
The ratio was about, animal:block;1:3
Wait long enough for the baby animal mobs to become adults.
Sometimes there will be animals outside of the fence pen.